The present invention relates to a system for detecting the presence of a pure signal in a measured discrete noisy signal, said detection being performed with a false detection probability below a predetermined false detection level. The invention relates to the detection of discrete signals, i.e. signals produced by discontinuous events. The invention particularly applies to the detection of corpuscular radiation, such as neutron or gamma rays.
The detection of a pure signal in a measured noisy signal assumes that it is possible to eliminate the noise in said measured signal. The invention relates to a detection process when the average or mean noise level is constant. This condition is substantially always realized for neutron radiation, because there are very few free neutrons in nature. It is also often realized at a given location for a certain period of time in the case of gamma radiation.
The detection of a particular corpuscular radiation, e.g. of the nuclear type, takes place in the presence of corpuscular noise having several causes. In the case of nuclear radiation, part of the noise is due to cosmic radiation and the other part is due to the natural radioactivity of the ground and rocks. Noise due to the detector used also exists. Cosmic radiation can vary over a period of time and the corpuscular noise due to the rocks and ground is essentially variable, as a function of the place where the measurement is performed. A particular radiation can consequently only be detected on knowing the noise or on having an adequate estimate thereof.
Information is stored for a limited time .DELTA.T. This information storage takes place in a discrete manner, when a particle is detected, the content of a memory is increased by one unit. In general, one or more parameters define the detected nuclear event. In the case where this event is only dependent on a single parameter, the latter can e.g. be the energy and this is the typical case of detecting a gamma radiation with the aid of an intrinsic germanium or a NaI scintillator associated with a photomultiplier. This single parameter can also be time, e.g. when measuring the transit time of a particle. The detection condition can also be a complex logic function dependent on the state of several detectors during a very short time interval, generally a few dozen nanoseconds, e.g. in the case of the detection of particles in a bubble chamber.
Normally, to know whether a signal has been detected with the aid of a storage performed during a time interval .DELTA.T, existing practice consists of evaluating by what value the count obtained exceeds the available noise estimate. This noise can be estimated in different ways. It can be known beforehand by a previously performed measurement in circumstances where certainty exists that there is no particular signal. It can also be evaluated during experiments by extrapolating or interpolating the detected signal levels in different intervals, e.g. of an energy type. For this purpose, there has to be an adequate counting rate in the intervals where certainty exists that there is no noise, in order to be able to make an estimate of the noise probability density in the intervals where it is wished to detect a particular radiation.
In a conventional manner, it is considered that there is a signal when the counting rate N is such that N&gt;B+K.sqroot.B, in which B is the estimated average noise level and K is a constant equal to a few units.
This known detection criterion is not satisfactory because it is of an empirical nature, the value of K being chosen in a non-determinist manner on the basis of previous experiments.